1. Field of the Invention
The present invention relates to an exhaust gas emission technology for a hybrid vehicle driven by an internal combustion engine and an auxiliary power source.
2. Description of the Related Art
Recently, there has been a demand for a reduction in fuel consumption of and emissions from internal combustion engines installed in motor vehicles. As a consequence, hybrid vehicles having two drive power sources, e.g., an internal combustion engine and an electric motor, have been pursued.
Known hybrid vehicles have incorporated an internal combustion engine, an electric power generator that is operated by drive power from the engine, a battery for storing electric power generated by the generator, an electric motor operated by electric power generated by the generator and/or electric power stored in the battery, wheels connected mechanically to a rotating shaft of the electric motor, and a drive power split mechanism that distributes drive power from the engine to the generator and to the rotating shaft of the electric motor.
When a load applied to this vehicle is within a low load range, for example, when the vehicle begins to move or during low-speed running, the engine is stopped and electric power from the battery is applied to the electric motor. Upon receiving electric power from the battery, the electric motor turns its rotating shaft. Therefore, the rotating shaft of the electric motor is turned by drive power generated by the electric motor, and the torque of the rotating shaft is transmitted to the wheels. Thus, in this case the hybrid vehicle runs only on the electric power from the battery.
When the vehicle load is within an intermediate range, e.g., during normal driving, the engine operates and the drive power split mechanism distributes drive power from the engine to the generator and the electric motor. Using the drive power distributed from the drive power split mechanism, the generator generates electric power. Power generated by the generator is applied to the electric motor which then turns its rotating shaft. In this case, the rotating shaft of the electric motor is turned by a combined drive power including power generated by the electric motor and power distributed from the drive power split mechanism. The torque from the rotating shaft is transmitted to the wheels. Thus in this case, the hybrid vehicle runs on power generated by the engine and on electric power generated using power from the engine.
When the vehicle load is within a high load range, e.g., during acceleration or the like, the engine is operated and the drive power split mechanism distributes power from the engine to both the generator and the rotating shaft of the electric motor. The power transmitted to the generator from the engine via the power split device operates the generator to generate. The electric power generated by the generator is combined with electric power from the battery and this combined power is applied to the electric motor to drive the rotating shaft of the motor. Therefore, the rotating shaft of the electric motor is turned by power generated by the electric motor combined with drive power distributed from the drive power split mechanism. Torque from the rotating shaft is transmitted to the wheels. Therefore, the hybrid vehicle runs on drive power generated by the engine, electric power generated using drive power from the engine, and electric power from the battery.
When the hybrid vehicle is in a decelerating or braking state, regenerative electric power generation is performed using torque transmitted from the wheels to the rotating shaft of the electric motor. That is, as the wheels and the rotating shaft of the electric motor are mechanically connected, torque is transmitted from the wheels to the rotating shaft of the electric motor during deceleration or braking of the vehicle operating the electric motor as a power generator. Thus, it is possible to perform generally termed regenerative electric power generation, that is, conversion of the kinetic energy transmitted from the wheels to the rotating shaft of the electric motor into electric energy. Electric power regeneratively generated by the electric motor is stored in the battery.
In the hybrid vehicle, when the battery must be charged or the engine needs to be warmed-up, the engine is started and drive power is transmitted from the engine to the generator via the drive power split mechanism to generate electric power.
Thus, the engine of the above-described hybrid vehicle operates efficiently and reduces fuel consumption.
With regard to vehicle-installed internal combustion engines, there are also demands to reduce harmful gas components present in exhaust gas, such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxide (NO.sub.x), and the like.
To respond to such demands, exhaust gas emission catalysts, such as three-way catalysts, oxidation catalysts, NO.sub.x -absorbing/reducing catalysts, selective NO.sub.x -reducing catalysts and the like, have been arranged in exhaust passages to treat or lessen harmful gas components contained in the exhaust gas.
Exhaust gas emission catalysts, as listed above, have predetermined activation temperatures (e.g., 300-500.degree. C.) above which they are able to lessen harmful gas components present in exhaust gas. That is, when the temperature of such a catalyst is below its activation temperature (e.g, at the time of cold start of the engine, or the like), it is difficult to sufficiently lessen harmful gas components present in exhaust gas.
At the time of a cold start of the engine, in particular, an increased amount of unburned fuel (e.g., HC) is present in the exhaust gas because an amount of fuel injected to the engine is increased above a normal level to improve the startability of the engine, to promote the warming-up of the engine, etc. However at this time, combustion in the engine becomes unstable. If the exhaust gas emission catalysts are not activated in this situation, relatively large amounts of unburned fuel are left untreated and are emitted into the atmosphere.
As a solution to the above-described problem, an engine exhaust gas removing apparatus is described in Japanese Patent Application Laid-Open No. HEI 4-194309. The exhaust gas removing apparatus includes a catalytic converter provided in an exhaust passage of an engine, a bypass passage connected to the exhaust passage which bypasses the catalytic converter, a filter chamber in the bypass passage for adsorbing unburned fuel components contained in exhaust gas below a predetermined temperature and for releasing adsorbed unburned fuel s when tho temperature is at least the predetermined temperature, a recovery passage connecting a portion of the bypass passage downstream of the filter chamber to a portion of the exhaust passage located near an inlet of the catalytic converter, a first open-close valve in a portion of the exhaust passage upstream of the connection to the recovery passage, a second open-close valve in the recovery passage, a third open-close valve in a portion of the bypass passage downstream of the connection to the recovery passage, and a flow regulator valve in a portion of the bypass passage upstream of the filter chamber.
When the catalytic converter of this system is not activated, the first open-close valve opens the exhaust passage, the second open-close valve closes the recovery passage, the third open-close valve opens the bypass passage, and the flow regulator valve is fully opened.
In this situation, all exhaust gas from the engine bypasses the catalytic converter and flows into the atmosphere via the bypass passage. When exhaust gas passes through the filter chamber provided in the bypass passage, unburned fuel contained in exhaust gas is adsorbed into activated carbon provided therein.
When the catalytic converter is activated, the first open-close valve opens the exhaust passage, the second open-close valve opens the recovery passage, the third open-close valve closes the bypass passage, and the flow regulator valve is driven to a predetermined opening. Most of the exhaust gas flows into the atmosphere via the catalytic converter, and the remainder flows into the bypass passage. The amount of exhaust gas introduced into the bypass passage flows into the atmosphere via the filter chamber, the recovery passage and the catalytic converter. Heat from exhaust gas passing through the filter chamber increases the temperature in the filter chamber to at least the predetermined temperature, so that unburned fuel adsorbed in the filter chamber is released and is led together with exhaust gas into the catalytic converter, wherein the unburned fuel is treated and lessened in amount.
If the load on the engine increases while unburned fuel contained in exhaust gas is being adsorbed in the filter chamber, that is, while the entire amount of exhaust gas from the engine is passing through the filter chamber, the exhaust gas temperature rises and the flow of exhaust gas increases, so that the amount of heat transferred from exhaust gas to the filter chamber increases. As a result, the temperature in the filter chamber rapidly rises, so that unburned fuel adsorbed in the filter chamber starts to desorb.
If the engine load increases immediately after the engine is started, the temperature in the filter chamber may rises to or above the predetermined temperature before a significant amount of unburned fuel has been adsorbed in the filter chamber. In that case, therefore, the adsorbing capability of the filter chamber is not fully utilized.
Furthermore, if the filter chamber temperature rapidly increases due to an increase in the engine load, the filter chamber temperature reaches the predetermined temperature before activation of the catalytic converter, so that unburned fuel released from the filter chamber may be let out into the atmosphere without being sufficiently treated.